Computed radiography (CR) and digital radiography (DR) imaging processes provide image content by converting received short wavelength x-ray energy to photoluminescent energy at higher spectral wavelengths, such as visible light energy. In CR, the energy from x-ray radiation is stored temporarily in a photostimulable storage phosphor medium for later release under an excitation light source and reading by an array of photodetectors. In DR, the energy from x-ray radiation can be converted directly to light energy as it is received; the emitted light energy from a scintillator layer in the DR device is then detected by a photodetector array that is adjacent to the scintillator layer.
Light scatter presents an obstacle to obtaining accurate pixellated data from either the CR storage phosphor plate or the DR detector scintillator layer. Some of the light energy that is stimulated from the light emitting material is not directed at angles that allow it to be directly sensed by detector circuitry, but radiates elsewhere within the image reading apparatus. This scattered light can contribute to image noise and degrade image contrast and overall image quality.
Various measures are taken to help prevent stray light from repeated reflection within the DR detector or CR reading apparatus, as well as to help keep ambient light from the detector circuitry. The image detection circuitry is typically protected from ambient light entry by design practices that provide covers, seals and gaskets, and other light-limiting features. Within the CR reading chamber or DR detector housing, non-reflective paints and coatings are typically provided, helping to absorb, rather than reflect, stray light from the photoluminescent materials themselves.
One inherent difficulty with any type of coated surface relates to reflectivity to electromagnetic radiation. Any smooth surface has been found to reflect light to some extent. Even light-absorbing paints and coatings exhibit some amount of reflection, unable to fully absorb incident light due to Fresnel reflection. By way of example, charcoal, normally considered to be a highly light-absorbent material, reflects as much as 4% of incident light. Even paints and coatings used for advanced aerospace imaging and measurement systems can exhibit reflectivity greater than about 0.5%. Within the confined space that is used for sensing stored or scintillated light from x-ray detectors, even very low levels of reflection can have a negative impact on image quality.